Allogeneic stem cell transplantation (alloSCT) can be life-saving therapy for malignant and nonmalignant diseases. T cells that accompany allogeneic stem cell grafts are vital for immune reconstitution, particularly in adults with little thymic function. Unfortunately, donor T cells also cause Graft-vs.-Host Disease (GVHD), the broad attack of donor T cells against recipient tissues. Therefore, all patients receive GVHD prophylaxis either via depletion of T cells from the allograft or through pharmacologic impairment of T cell function. GVHD and the infectious complications of immunosuppression used to prevent it are barriers that block the more widespread application of alloSCT. We have shown that GVHD requires functional host antigen presenting cells (APCs) and that their depletion with alloreactive natural killer cells prevents GVHD in both mice and humans. This represents a novel form of GVHD prevention and treatment. We have therefore been developing immunotoxins that deplete APCs and have found that immunotoxins against MHCII and CD11c deplete murine dendritic cells in vivo and decrease GVHD when administered prior to alloSCT. However, neither MHCII nor CD11c are ideal targets due to their tissue distribution and inefficient internalization. We hypothesize that c-type lectins receptors (CLRs) will be superior targets because they a) efficiently internalize and localize to subcellular compartments critical for toxin delivery and b) are expressed on APCs. We will test this hypothesis with the following Aims: Aim 1. We will test the hypothesis that immunotoxins directed against CLRs will efficiently deplete murine DCs in vivo. To do so we will test the efficacy of 2 immunotoxins: The first targets the human mannose receptor (hMR) and is based on a proprietary human anti-hMR antibody. We will test this reagent in mice transgenic for hMR. The second targets murine DEC205, a CLR expressed on CD8+ dendritic cells. Aim 2. We will perform toxicity studies these immunotoxins. Aim 3. We will test the hypothesis that immunotoxin depletion of hMR+ and DEC205+ cells will decrease GVHD. Relevance to Public Health. Currently many patients with cancer and inherited disorders of blood are not offered hematopoietic stem cell transplantation because of the toxicity caused by immune cells in the donor that attack the host. The goal of our work is to prevent this attack, thereby making this therapy safer and more widely applicable [unreadable] [unreadable]